It is often desirable in drilling oil wells to enlarge the diameter of the hole being drilled at some substantial distance below the surface. This may be needed, for example, to provide a sufficient annulus between casing and the formation being drilled to permit cementing the casing in place in the hole.
When this is desired, an expandable underreamer is lowered through the well with its cutter arms retracted. At the desired elevation, the arms are extended and the diameter of the hole is reamed to be larger than the hole through which the underreamer was lowered. When the desired length of hole has been reamed, the arms are retracted and the underreamer withdrawn from the hole.
A variety of underreamers have been designed for this purpose. In some of them the arms are moved from their retracted to their extended position by application of hydraulic pressure as drilling mud is pumped down the drill string supporting the underreamer. In others the arms are extended by weight of the drill string when the assembly including the underreamer is set down against the bottom of the hole. The underreamer described in U.S. Pat. Nos. 3,817,339 and 3,949,820 is exemplary of such an underreamer. The subject matter of these patents is hereby incorporated by reference.
The patented underreamer is desirable for a number of reasons. It has two extendible arms instead of the usual three, which means that the arms and body can be stronger and larger cutters can be mounted on the arms. This can mean that a harder rock formation can be reamed, or that the rate of penetration of the underreamer may be greater, or the underreamer drills further since larger bearings and slower speeds increase drilling life. The patented underreamer provides for flow of fluid past the cutter arms so that a drill bit or the like can be placed below the underreamer and a hole concurrently drilled and reamed.
As mentioned, this underreamer uses set down weight for actuating the extendible arms. It is desirable for greater versatility to extend the arms by application of hydraulic pressure of drilling mud. It is desirable to obtain this advantage without impact on the other advantages this underreamer has.